The use of motors in bore hole drilling, especially drilling for oil and gas but also in mining operations, has been a standard procedure in the art. Such motors are employed to rotate drills for boring in the earth both for forming a bore hole and also for coring. The motors are also useful in oil field operations, such as tube cleaning, milling operations, cement drilling and other operations where it is desired to rotate a rod at the end of which a tool is to be rotated. We refer to such motors as in-hole drills when designed to be run at the end of a pipe and adjacent to the drill bit. In the usual case, the rotor of the motor and the drill bit rotate with respect to a stator which, in turn, is connected to a conventional drill string composed, in the case of the drilling of well bores, of a "kelly," drill pipe and drill collar as required. This string extends to the surface with the kelly mounted in the rotary table. Where the in-hole motor is a hydraulic motor used as an in-hole motor used in drilling, the liquid is the usual drilling fluid, i.e., mud or gas. It serves its usual function in the drilling operation, returning to the surface carrying the detritus, i.e., cuttings resulting from the drilling operation. However, in this combination, the circulating mud has an additional function and that is to supply the hydraulic power to operate the hydraulic motor.
One of the primary problems resides in the design of the bearing system which will permit operations for periods of economic length.
Because of the failure of bearing seals in the prior art, it has been conventional to employ bearings without seals and to rely on a part of the circulating mud to pass through the bearings to lubricate them. Such bearing systems are shown in E. P. Garrison et al, U.S. Pat. No. 3,516,718 issued Jan. 23, 1970.
When mud lubricated bearings are employed with motors of the helicoidal type, such as have been employed in the prior art in-hole motors, problems arise with respect to limiting the flow of mud through the bearings and problems arise from the eccentric motion of the rotor. Such motors are shown in Clark U.S. Pat. No. 3,112,801, patented Dec. 3, 1963, and have been widely distributed by Smith International, Inc. under their registered trademark "Dyna-Drill". Such motors are described in the article by H. M. Rollins, "Bit Guiding Tools Provide Better Control of Directional Drills," World Oil, June 1966, pages 124-135, and are also discussed below.
The prior art solutions for limiting the by-pass of mud through the bearings are shown in the Garrison patent. These include the provision of a grooved rubber radial bearing which also acts as a flow restrictor to limit the fluid by-passing through the bearings so as not to rob unduly the main flow through the bit nozzles required to provide the necessary flow to remove the cuttings.
Since the rotor of the motor rotates in an eccentric manner, it is necessary to convert this motion into a true rotation about a fixed axis so that the bit may be rotated in the proper manner. This is accomplished by connecting the end of the rotor to a connecting rod by means of a universal joint and connecting the connecting rod to a drive shaft by means of a second universal joint.
Problems have arisen in such prior art combination. The rubber radial bearings, which even in the first place due to molding limitations do not act adequately to restrict the amount of by-pass, deteriorate in use and result in premature failure. This failure includes erosion of the bearing passageways whereby the grooves are washed out. The rubber is torn away from the sleeve to which it was bonded. Bearing failure occurs, in part, due to the excessive flow of mud through the bearings resulting in an excessive erosion of the bearings. An additional difficulty is that the excessive by-pass requires an increase in the volumetric rate of mud flow into the system in order to maintain the required nozzle velocities and pressure drops across the bit nozzles to move the mud cuttings to the surface, and aid in the cutting action.
The percentage of the fluid by-passed, even with newly formed radial rubber bearings, it excessive because it is difficult to mold such bearings to form passageways through the bearings that will have the desired flow resistance and yet provide a suitable bearing surface which will not have excessive frictional resistance. The erosion of the mud is also a problem. It is to be recognized that pressures drop between the stator discharge to the annulus exterior of the drill may be of the order 200 to 1500 pounds per square inch and a volumetric rate of flow from 50 to about 600 gallons per minute, depending upon the depth, nature of the mud, size of the tool, designs of the nozzles of the bit.
The pressure drop and volume rate of flow of the mud through the stator depend on the horsepower requirement and drill and rpm of the drilling effort. This establishes the gallons per minute of mud that must be circulated. The mud input pressure is fixed by the total pressure drop through the drill string, the hydraulic motor, bit nozzle and annulus pressure drop. The volume by-passed through the bearings is subtracted from the flow through the nozzles. The pump must provide for sufficient input to supply the required flow rate and pressure drop. Mud by-passed through the bearings discharges into the hole annulus. It adds to the mud input required to supply that required to be passed through the nozzles and adds to the velocity of the mud in the annulus. The bit manufacturer usually supplies the nozzle pressure drop to give the required lifting effect and cutting action. Furthermore, the depth to which a well may be serviced by a given pump assembly and therefore the limit of bit advance depends on the permissible horsepower required to move the mud through the motor to and through the bit nozzles and return the cuttings to the surface. Any additional demand on the pump, required to supply excessive by-pass is a limitation on the depth to which a given drilling rig can go.
It is difficult to build a rubber bearing which is so finely tuned as to meet these parameters and not permit an excessive flow through the bearings. Furthermore, as has been stated above, pressure drops tend to erode the passageways in the rubber bearing so that they do not for long retain their original cross-sectional areas.
Further, while the universal joints do a fairly good job in the case of the helicoidal motors of converting the eccentric motion of the rotor to a rotary motion, there remains a residual force on the drive shaft which is transverse to the axis of rotation. This transverse force is periodic in direction, reversing itself on each reversal of the eccentric motion. Additionally, when drilling in steeply dipping formations or in drilling out dog legs, or in drilling deviated holes, particularly when using bent subs at the connecting rod, a thrust is encountered at the bit which is transverse to the bit axis. The result is a working of the rubber of the bearing.
An additional problem with rubber bearings is in the influence of the geothermal effect. The temperature in the bore hole may range up to several hundred degrees above ground temperature, depending on the depth. This adds to the heat developed from friction, which due to the low heat conductivity of rubber is not readily carried away by the circulating mud. Despite the cooling effect of the by-pass fluid, this temperature taken together with the frictional heat of the bearing, and the hysteresis of the rubber resulting from the cyclic transverse thrust of the drive shaft, all operate to impair the physical properties of the rubber. Failure of the rubber bearing is not uncommon.
An additional problem arises, as we have found from the failure in the bond between the rubber and the sleeve to which it is bonded. We have observed large pieces of the rubber bearing that have been torn away from the sleeve in use. We have solved the problem arising from deficiencies of the rubber material by employing instead of a rubber, a radial bearing of hard, rigid metal.
In the co-pending application Ser. No. 388,586, by employing material having a hardness greater than the "Sand" or other solid materials in the circulating mud, we avoid the problem arising from the erosion of the by-pass passageways. The bearings may be formed with restricted passageways which may be accurately formed to be having, and have, a hardness greater than the "sand" or other solid materials in the circulating mud, we avoid the problem arising from the erosion of the by-pass passageways. The bearings may be formed with restricted passageways which may be accurately formed to be metering passageways for the mud. Being made of hard metal, problems arising from maintaining the integrity of the radial bearing are avoided. Being made of rigid metal, the residual oscillatory forces transverse to the drive shaft are resisted and a true axial rotation is made possible. Being made of metal of good heat conduction, frictional heat will not build up in the bearing parts and the cooling effect of the mud will be effective to cool the bearing parts.
In our invention as described in said co-pending application, accurate metering characterists are achieved. The mud is permitted to wet the rubbing surfaces and act as a lubricant.